1. Field of the Invention
The present invention relates to a semiconductor laser device for use in an optical information-processing apparatus, an optical measuring apparatus, or the like, and more particularly to a semiconductor laser device which has a double-hetero structure having an optimal layer thickness.
2. Description of the Related Art
In recent years, development has been made of short-wave semiconductor lasers for use in high-density optical disk systems, high-speed laser printers, bar code readers, or the like. Of these semiconductor lasers, the InGaAlP laser, which emits a beam having a wavelength of 0.6 .mu.m (read-light range), can be used in place of the existing He-Ne gas laser for a variety of uses. Therefore, much attention is paid to the InGaAlP laser in the fields of optical data processing and optical measuring, since it can be a small, light, and low power-consumption light source. This semiconductor laser must have as good characteristics and reliability as those of the conventional GaAlAs laser, if it is to be used practically.
GaAlAs lasers of various structures have been developed. Each of these lasers has specific structural parameters, and has desired characteristics by virtue of these structural parameters. In other words, optimal structural parameters are known for each type of a GaAlAs laser. By contrast, no structural parameters which seem the most desirable for an InGaAlP laser have been proposed. This is because the InGaAlP laser is rather a novel type of a semiconductor laser, and the physical properties of experimental InGaAlP lasers have yet to be evaluated completely.
The experiments, which the Inventors hereof have conducted, show that the oscillation threshold value of an InGaAlP laser greatly depends upon the thickness of its active layer. The experiments also suggest that an InGaAlP laser cannot be sufficiently reliable unless its active layer has an optimal thickness. Hence, it is required that the active layer of an InGaAlP laser have an optimal thickness.
An InGaAlP material is greatly different from a GaAlAs material in thermal resistivity. For example, GaAs and Ga.sub.0.6 Al.sub.0.4 have the thermal resistivities of 2K cm/W and 8K cm/W, respectively, whereas In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P has the thermal resistivity of 17K cm/W. When the cladding layer of a semiconductor laser is made of InGaAlP, the laser has a high thermal resistance. Thus, during the operation of the laser, its active layer will be heated to a high temperature, and the threshold current of the laser will inevitably increase. Therefore, the InGaAlP laser has unstable thermal characteristics and inadequate reliability.
As is generally known, the characteristics of a GaAlAs laser are more influenced by the stripe width and the cavity length than by the other structural parameters. (See W. B. Joice et al., Journal of Applied Physics, Vol. 46, pp. 855-862, 1975.) In the case of an InGaAlP laser, the thickness of the cladding layer is one of the structural parameter which greatly influence the thermal resistance of the laser. The cladding layer of most conventional semiconductor laser has a thickness of 1 .mu.m or more, being thick enough to prevent the waveguiding mode from being affected by the substrate or the contact layer. In the case of an InGaAlP laser, as the results of the experiments conducted by the inventors hereof have revealed, when the cladding layer is 1 .mu.m or more thick, the threshold current for CW operation increases too much in contrast with the case for pulsed operation. Consequently, the InGaAlP laser fails to have good thermal characteristics or sufficient lifetime.
Various methods of reducing the thermal resistance of the InGaAlP laser have been proposed. Japanese Patent Disclosures No. 61-280694 and No. 62-81783 disclose an InGaAlP laser whose cladding layer consists of two layers, the outer one of which is made of material having a low thermal resistivity. Japanese Patent Disclosures No. 62-51282 and No. 62-51283 disclose an InGaAlP laser which comprises a cladding layer formed of a superlattice. Either InGaAlP laser has a complex structure, and many interfaces involve in growing crystals. Hence, many manufacture parameters must be controlled to manufacture the laser. Consequently, the InGaAlP laser cannot be manufactured in a high yield or exhibit sufficient reliability.
As has been pointed out, the structural parameters of a InGaAlP semiconductor laser have yet to be optimized. In particular, the thickness of the active layer is not optimal, and the oscillation threshold value of the laser is excessively great. Further, since the thickness of the cladding layer is not optimal, the InGaAlP laser fails to have good thermal characteristics or sufficient reliability.
When a semiconductor laser is used as a light source in an optical information-processing apparatus, its transverse mode must be controlled. Known as an InGaAlP laser, whose transverse mode can be controlled, is a ridge-stripe SBR laser. (See extended abstracts, 19th Conf. Solid/state Devices and Materials, Tokyo, 1987, pp. 115-118.) This laser can emits a beam in fundamental transverse mode, owing to its specific structural parameters. However, the structural parameters of the ridge-stripe SBR laser have not been optimized so as to reduce astigmatism or to stabilize the transverse mode. The optical characteristics of the laser greatly changes in accordance with the compositions forming the lasers, the thickness of the active layer, the width of the stripe, and the like. For example, no difference can be made between the layers in effective refractive index, with respect to the horizontal direction. Consequently, the ridge-stripe SBR laser functions almost in the same way as a gain-waveguide laser, whereby the astigmatism inevitably increases. Further, it operates in a high-order mode unless the stripe width and some other structural parameters are optimized. If this is the case, the current-output characteristic drastically alters in a low-power region, and the laser cannot be used in some types of optical disk apparatuses. No quantitative analysis has been made of the dependency of the current-output characteristic upon the structural parameters. Therefore, no ranges are known within which the structural parameters should fall so that the ridge-stripe SBR laser may have a good current-output characteristic.
The inventors hereof have been conducting researches in order to provide a semiconductor laser which has an InGaAlP layer and a GaAs layer, both formed by MOCVD method, and a single fundamental mode. The inventors have found it that a semiconductor laser of the conventional structure, whose comprises a cladding layer having stripes and current-blocking layers extending beside the stripes can hardly operate reliably or the manufactured with a high yield. This is because a leakage current flows in the current-blocking layers, disabling the laser to emit a sufficiently intense beam. The inventors have also found that selenium, which is the n-type dopant contained in the current-blocking layers, diffuses into the p-type cladding layer formed on the active layer, inevitably rendering the cladding layer n-type, and further diffuses into the n-type cladding layer formed below the active layer. In consequence, the current-blocking layers can no longer block a current to a sufficient degree, disabling the laser to emit a sufficiently intense beam.